Textured surfaces are used on web conveyance machine rollers to relieve the air that is entrained between the roller and the moving web. This allows the webs to be run at higher speeds while maintaining traction between the web and the roller surface. In the present invention a textured surface is defined as a surface which is three dimensional and includes interconnected plateaus or craters. Textured surfaces can also include rounded peaks and valleys.
The conventional measurement method to determine the texture of a surface is to run a stylus across the outer surface at some position on the circumference. From this measurement a numerically estimated standard roughness parameter is determined. This parameter can be expressed as average roughness, RMS or 10 point, and from this measurement a characterization of the resulting traction is inferred. The main drawback of the stylus method is that the roughness values measured are representative of a small section taken from the tracing stylus and not of the whole surface. In addition, none of the estimated parameters provides true characteristics of the surface venting. Since surface venting is the most reliable way to determine the traction of the roller, the prior art off-line methods do not properly measure the roller characteristics. Even advanced optical measurement systems, which can show three dimensional irregularities, have the limitation of not being able to characterize the surface venting.
Dynamic traction testing is one on-line method used to characterize surface venting of the surface of the roller. The other known on-line method is by measuring the air film thickness between two moving surfaces under various operating conditions. Both these methods require a web conveyance machine for the test to be conducted.
These limitations of the prior art have resulted in difficulties in producing surfaces with consistent and repeatable traction and venting characteristics. This problem is illustrated in FIG. 1 which shows results of surface venting characteristics of several rollers. All of the rollers met the specifications of the standard roughness parameter using the stylus method. The surface venting was then characterized through dynamic traction testing. It can be seen that although the rollers were manufactured to meet the same specifications, the surface venting varied significantly from roller to roller.
Thus, there is a need to develop a device which can accurately characterize the surface venting of the roller and, therefore, the roller's traction without requiring a web conveyance machine to run a cumbersome test. The present invention describes a method and apparatus which allows one to accurately characterize the surface venting and traction of a textured roller surface without the use of web conveyance machinery.